Koloktos followed by Volvagia. In the 2D games it would probably be Agahnim.
How could I forget!!
Jesus Christ, that boss fight made me rage so hard I actually threw my Wii Remote across the room and punched the wall. Maybe I blocked it out to keep from going insane.
Yeah, I got a Game Over twice because of him. Not because I died, but because he got to the top. That was pretty much the only difficulty spike in the entire game.